Obesity results from an imbalance between food intake and energy expenditure, such that there is a net increase in fat reserves. Excessive food intake, reduced energy expenditure, or both may contribute to this in balance. Clinically, obesity is defined relative to body mass index (BMI), a measure of body weight to body surface (kg/m2). A normal BMI is considered to be in the range from greater than 30 kg/m2. A BMI in the range from 25-30 is considered overweight, while obese is classified as having a BMI >30. Obesity is classified into 3 subcategories: Class I—moderate; Class II—severe; and Class III very severe.
It is well established that patients with elevated BMIs are at increased risk for a variety of diseases including hypertension and cardiovascular disease, kidney disease, diabetes, dyslipidemia, sleep apnea, and orthopedic problems. Obesity has become pandemic in the U.S. with a prevalence exceeding 30%. The increased demand on health care resources due to obesity and the health problems associated with it are estimated in the U.S. to exceed $200 billion annually.
Appetite and satiety, which control food intake, are partly regulated by the hypothalamus region of the brain. Energy expenditure is also controlled in part by the hypothalamus. The hypothalamus regulates the autonomic nervous system of which there are two branches, the sympathetic and the parasympathetic. The sympathetic nervous system generally prepares the body for action by increasing heart rate, blood pressure, and metabolism. The parasympathetic system prepares the body for rest by lowering heart rate, lowering blood pressure, and stimulating digestion. Destruction of the lateral hypothalamus results in hunger suppression, reduced food intake, weight loss, and increased sympathetic activity. In contrast, destruction of the ventromedial nucleus of the hypothalamus results in suppression of satiety, excessive food intake, weight gain, and decreased sympathetic activity.
The splanchnic nerves carry sympathetic neurons that innervate the organs of digestion and the adrenal glands, while the vagus nerve carries parasympathetic neurons that innervate the digestive system, and as experiments involving hypothalamic destruction have shown, those neurons involved in the feeding and weight gain response.
Experimental and observational evidence suggests that there is a reciprocal relationship between sympathetic nervous system activity and food intake, with an increase in sympathetic activity generally leading to a reduction in food intake. These effects are mediated by specific neuropeptides (e.g., neuropeptide Y, galanin) known to decrease sympathetic activity, which in turn triggers an increase food intake. Other peptides such as cholesystokinin, leptin, and enterostatin, increase sympathetic activity, thus decreasing food intake. Ghrelin is a peptide that is secreted by the stomach and which is associated with hunger. Peak plasma levels of this peptide occur just prior to mealtime, and ghrelin levels are increased after weight loss. Sympathetic activity can suppress ghrelin secretion. PYY is a hormone released from the intestine that also plays a role in satiety. PYY levels increase after meal ingestion. Sympathetic activity can increase PYY levels. Similarly, drugs such as nicotine, ephedrine, caffeine, subitramine, dexfenfluramine, which lead to an increase in sympathetic activity, also reduce food intake.
Appetite is stimulated by various psychosocial factors, but also by low blood glucose levels, as well as mechanical receptors in the gastrointestinal tract. For example, cells in the hypothalamus that are sensitive to glucose levels are thought to play a role in the hunger response, increasing the sensation of hunger as glucose levels decrease. In response, activation of the sympathetic nervous system leads to an increase in plasma glucose levels. Similarly, a feeling of satiety can be promoted by distention of the stomach and delayed gastric emptying. Sympathetic nerve activity reduces gastric and duodenal motility, causes gastric distention, and can increase contraction of the pyloric sphincter, which can result in distention and delayed emptying of the stomach contents.
The sympathetic nervous system also plays a role in energy expenditure and thus the tendency towards obesity. Genetically inherited obesity in rodents is characterized by decreased sympathetic activity in adipose tissue and other peripheral organs. Catecholamines and cortisol, which are released by the sympathetic nervous system, cause a dose-dependent increase in resting energy expenditure. In humans, a negative correlation between body fat and plasma catecholamine levels has been reported.
Over- or under-feeding lean human subjects can have significant effects on sympathetic nervous system activation and energy expenditure. For example, weight loss in obese subjects is associated with a compensatory decrease in energy expenditure, which promotes regaining previously lost weight, a common problem that limits the effectiveness of classic diet and exercise weight loss programs. Conversely, drugs that activate the sympathetic nervous system, such as ephedrine, caffeine, or nicotine, are known to increase energy expenditure. Smokers are known to have lower body fat stores and increased energy expenditure relative to non-smokers. Weight gain is a commonly reported consequence of quitting smoking. The sympathetic nervous system also plays an important role in regulating energy substrates such as fat and carbohydrate. Metabolism of glycogen and fat, needed to support increased energy expenditure, is increased by sympathetic activation.
Animal research shows that acute electrical activation of the splanchnic nerves causes a variety of physiologic changes. Electrical activation of a single splanchnic nerve in dogs and cows causes a frequency dependent increase in catecholamine, dopamine, and cortisol secretion, and circulating plasma levels of these compounds that lead to increased energy expenditure can be achieved. In adrenalectomized pigs, cows, and dogs, acute single splanchnic nerve activation causes increased blood glucose levels with concomitant reduction in liver glycogen stores. In dogs, single splanchnic nerve activation causes increased pyloric sphincter tone and decreased duodenal motility, reducing the rate of food passage through the gastrointestinal tract, and in turn leading to a sensation of satiety. Moreover, sympathetic, and specifically splanchnic nerve, activation can result in suppression of insulin and leptin hormone secretion.
First-line therapy for obesity treatment is typically behavior modification involving reduced food intake and/or increased exercise. However, these approaches frequently fail and behavioral treatment is commonly supplemented by administration of pharmacologic agents known to reduce appetite and/or increase energy expenditure. Commonly used pharmacologic agents include dopamine and a dopamine analogues, acetylcholine and cholinesterase inhibitors. Pharmacologic therapy is typically delivered orally. However, treating obesity with drugs frequently results in undesirable side effects, including systemic effects such as tachycardia, sweating, and hypertension. In addition, tolerance can develop such that the response to the drug is reduced even one higher doses are used.
More radical forms of therapy involve surgical intervention. In general, these procedures are designed to reduce the size of the stomach and/or reroute the gastrointestinal tract to substantially avoid the stomach, the net effect being either a reduction in the amount of food that can be consumed before feeling full, reduction in absorption, or both. Representative procedures include gastric bypass surgery and gastric banding. While these procedures can be effective in treating obesity, they are highly invasive, require significant lifestyle changes, and may have significant postoperative complications.
More recent experimental forms for treating obesity have involved electrical stimulation of the stomach (gastric pacing) and the vagus nerve (parasympathetic stimulation). These therapies make use of a pulse generator to electrically stimulate the stomach or nerves via implanted electrodes. The object of these therapies is to reduce food intake through the promotion of satiety and/or reduction of appetite. For example, U.S. Pat. No. 5,423,872 (Cigaina) discloses a method for treating eating disorders by electrically pacing the stomach. Similarly, U.S. Pat. No. 5,263,480 (Wernicke) discloses a method for treating obesity by likely activating the vagus nerve. However, while these methods are suggested to be effective in reducing food intake, neither is believed to affect energy expenditure.